Transmission system for telephony



Nov. 1, 1932. E. K. SANDEMAN TRANSMISSION SYSTEM FOR TELEPHONY Filed Dec. 12, 1930 r v FIG] WEE ERNIE." SNUkN-NQS 200 400 600 800 I000 I200 I400 I600 l8002000 220O 2400 CYCLES PERSECOND FIG. 2

CYCLES PER SECOND INVENTOR EDWARD K.SANDEMAN z; A! :ORNEY Patented Nov. 1, 193 2 eairssrarss PATENT OFFICE EDWARD KENNETH SANDEMAN, F ALDWYGI-I, LONDON, ENGLAND, ASSIGNOR TO INTER- NATIONAL STANDARD ELECTRIC CORPO Tron or DELAWARE.

, TRANSMISSION sYs'rEM non TELEPHONY Application filed December 12, 1930, SerialNo. 501,774, and in Great BritainjJ'anuary 7, 1930. 1

, This inventionrelates to thevtransmission of telephone signals overa line whichgives rise to distortion resulting from interference ornoise, and one object is to provide such a system wherein, having regardto these conditions, it may be so designed as to ;be used on a maximum energy level vfor a given quality of transmission. 7

According to one feature of the invention, a system, is provided wherein adjustments are made which produce for any elemental band of frequencies approximatelythe same number of overloads per unit of time.

I and the spectrum of received noise interference, the levels to whichthe speech energy sinks at each frequency at the receiving end of the cable, are of such magnitude that the rate of change in degradation of intelligibility of speech (due to noise) due to changing the noise level by equal infinitesimal amounts in any equal elemental frequency region, tends to zero. 7

Other features of the invention will be disclosed'in the. following description haying reference to the accompanying figures in which: 7 i v V I Fig. 1 is a curve connecting frequency in cycles per second as abscissa with interferin effect in decibels as ordinates.

Tig. 2 is a curve connecting frequency (f) v in cycles per second as abscissae with decibels (d) as ordinates."

In the firstembodiment, a transmitting amplifier is employed which has such a gain frequency characteristic that the average number of overloads. per minute contributed by frequency bands of equal width is approximately indepen-dent of the location of narrow, or new onx, 1v. Y., A coRronA- the band. The resultanttransmitted specfrequency characteristic that. the overall attenuation through the systenntransmitting'" amplifier, cable and receiving amplifier,- s

independent of frequency, or conforms to any predetermined attenuation frequency curve. The galn frequency curve of, the transmitcan" ting amplifier is in this caseindependent of the received noise spectrum. a V v This will be apprec1ated on consideration of the fOliOWlIlg:

If the nature of speech were such that only one frequency component were present at any one time, then it would evidently be desirable to transmit that one frequency at maximum power, and, if overloading were equally deleterious to intelligibility in every part of the frequency range, the optimum gain frequency characteristic for the transmitting amplifier would be one which made the probability of overloading the same in every part of the frequency range. The I transmitted speech spectrum would then be quite independent of the attenuation ofthe cable and the noise spectrum. In default of-exact knowledge of the nature of the interference spectrum an equal overloadspectrum would be the spectrum which on the average might'be expected to give the best performance because the am in transmitted level in any one part of the frequency range expressed in (Z?) in departing from an equal overload spectrum is always less than the corresponding loss which must be introduced in some other part of the frequency range also expressed in db. When more than one frequency occurs at one-time their amplitudes add directly at the peaks so that an increase in the" amplitude. of any one frequency-can on-lybeaccomplished by reducing the amplitude of the other frequencies present, the peak beinglimited by the power hanw dling capacity of the transmitting valves If two frequencies occur 'at atime each of the same amplitude so that their sum is the maximum permissible peak, then by cutting down the amplitude of one to a quarter,

that is by 12 db, the amplitude of the other may be increased by 1.25 times, that is, by 5 (Z12. The loss in transmission on the one fre quency is considerably greater than the gain in transmission on the other.

Method of producing an equal overload spectrum This involves a knowledge of the maximum instantaneous peak energy of speech, defined by any contour line of overload, corresponding to a given number of overloads per minute on a frequency band of given width, situated anywhere in the frequency range. This may for convenience be expressed in decibels below any arbitrary volume level by the amplification which must be applied on any band alone (all other bands being suppressed) to give adefinite band, the gain of the amplifier gives i number of overloads per minute. This results in a separate contour line of overloading, for any chosen number of over loads per minute on one band, plotted on a plane ofdecibels and frequency. If the en tire range is divided into bands and the tolerable number of overloads is L per minute on normal speech for the Whole range then it is proposed to choose he contour line corresponding to overloads per minute as that representing the maximum instantaneous peak energy of speech, though it is not intended to restrict the scope of this invention to this particular feature. In practice it is not expected that the shape of the contour lines will vary appreciably.

The maximum instantaneous peak level at any point in a circuit, in any band, may be obtained by supplying the speech from that point in the circuit to an amplifier system incorporating a band filter passing only that range, and terminating in 1. every band the ordinate of the overload spectrum of speech corresponding to the number of overloads per minute chosen, and re ated to the level equivalent to the setting of the peak voltmeter or volume indicator.

Thus in Fig. 2 S is the arbitrary overload level as would be represented by the overload level of the measuring valve, and the curve Q, is obtained by plotting negatively from S the mean amplification which must be given to each element a, b, 0, etc., of the speech band to produce in that element the predetermined number regard is taken of the fact that overloading" at the highest frequencies in the range is tolerable since their harmonics are not reproduced and they do not mask the low frequencies when they overload.

Method 07 producing cm equal overload intol- Zigz'bz'lity loss spectrum The procedure for obtaining the above spectrum is somewhat similar to that for obtaining an equal overload spectrum. By means of filters or equalizers part of the spectrum of a speech wave at a given point in a circuit is amplified more than the rest so that any overloading which occurs in the last valve of the amplifier may be considered to be definitely due to that part of the spectrum only. After passing the overloading point the Whole speech is passed through an equalizer Which makes the overall response of the circuit normal i. e. as would result in practice. By articulation tests a curve is plotted for each band between gain from the selected point on the circuit to the overloading point, and mid band frequency. From these, articulation contour lines of gain against mid band frequency, for constant articulation, are then plotted. An equalizer is then inserted after the selected point on the circuithaving an attenuation frequency characteristic of the same shape as the contour line of gain against mid band frequency corresponding to zero reduction of articulation due to overloading.

The speech output from the equalizer will then be an equal overload intelligibility loss spectrum as defined above. 7

in a further modification the transmitting amplifier has such a gain characteristic that, having regard to the attenuation frequency characteristic of the cable, and the spectrum of received noise interference (the overall gain or loss of the cable system, comprising transmitting amplifier, cable, and receiving amplifier conforming to any predetermined dependency on frequency), the levels to which the speech energy sinks at each frequency at the receiving end of the cable, are of such magnitude that the change in degradation of intelligibility of speech (due to noise) due to changing the noise level by equal infinitesimal amounts by the method described below, in any equal elemental frequency region, tends to zero. The method of reducing the noise level in any elemental frequency region is here considered to comprise increasing the transmitted power in that region by the amount by which the noise level in that region is to be lower, a corresponding reduction in transmitted power being made uni formly over the remainder of the range so that maximum intelligibility is obtained under the new condition. The method of determining the gain characteristic of the transmitting amplifier is by a series of articulation measurements with different gain-frequency characteristics for the transmitting and receiving amplifier, the overall attenuation at each frequency being kept constant.

A preliminary gain characteristic is first derived from the following (a) The maximum permissible power input at the transmitting end.

(6) The peak energy spectrum of speech at the input to the amplifier taking the con- L tour line corresponding to a overloads per 7 (Fig. 1) can be derived showing the relative interfering effect (as ordinates) of each 100 cycles noise band. This is then expressed in decibels relative to the ordinate at 1000 cycles, taken as zero, in such a way that noise voltages greater than the 1000 cycles noise voltages are plotted positively, noise voltages smaller than the 1000 cycles noise voltage are plotted negatively: this curve will be called the interference curve. The attenuation frequency curve of the cable is then plotted as positive values in decibels and the interference curve is added to it. The resultant curve gives a first approximation to V the slope of the gain characteristic'of the transmitting amplifier. If this gives a maximum gain at or near the frequency where the speech spectrum at the input of the trans mitting amplifier is also a maximum,then it L energy spectrum .(contour line of overloads per minute) at the frequency if maximum gain an the ordinate of the speech energy spectrum at the frequency of mimmum gain, expressed in decibels. The gain slope having been determined, the optimum absolute value of gain is found by experiment, by adding or subtracting distortionless gain on a resistance potentiometer.

It is evident that such instructions do not specify any one curve exactly but only ensure that the first curve tried shall lie in a certa n locality. Having in the above manner decided on the first trial gain curve for the.

transmitting amplifier, an amplifier having such a curve is made up, and also a corresponding receiving amplifier giving with the cable an overall attenuation of preassigned form. An articulation test is then made on the system comprising a series of complementary changes made in the gain character istics of transmitting and receiving amplifiers, keeping the overall attenuation at each frequency unaltered, each change being followed by an articulation test. Thus an elemental band of frequencies lying within the speech range is selected and the intelligibility at a particular energy level of said band is naeasured with the energy level of the remainder of the speech frequency range at a particular value. band constant, the level of the remainder of the frequency range is varied above and be low the condition giving maximum intelligibility to determine the condition giving maximum intelligibility. peated with the elemental band at a succession of different energy levels above and below the condition giving a maximum value of maximum intelligibility to determine the exact conditon giving maximum intelligibility. The condition of level ofthe elemental-frequency band and of therest of the frequency range which gives maximumintelligibility is determined from the readings, and this gives the relationship between the level of the ele mental band and the level of the rest of the frequency range for producing the maximum intelligibility. A second elemental band of frequency is then selected and while maintaining the relation between the first elemental band and the remainder of the speech frequency range, as was determined in the first experlment, the process is repeated'as before and a condition is obtained for the second elemental band 'giving'maximum :intelligi-" Keeping the level of said I The above is then reno I bility. The above process is repeated until as many elemental portions of the speech band have been considered as are thought to be necessary. Preferably the entire frequency range Would be covered. The measurement may then be recommenced with respect to the first elemental band and then each one in turn as before. To reach an absolute maximum of intelligibility Would require an infinite number of tests, but it has been found that a useful approximation is reached by using a finite number of tests, the number depending upon the quality of transmission desired and the importance of the circuits.

It is evident that this method carried to its logical conclusion Will produce gain characteristics, and a resulting noise level, Which are such that any complementary change in gain of finite value in either direction (and therefore change of noise level) Will produce a lowering of the intelligibility or converse lyv an increase in the degradation of intelligibility due to noise. In other Words the change of degradation of intelligibility With change in noise level (as carried out) is zero and the intelligibility of the circuit is the maximum possible attainable.

The invention is applicable to submarine telephone cable transmission.

What is claimed is:

1. The method of determining the characteristic of a Wave translating device for producing in a line a favourable energy level for the frequencies it is desired to transmit thereover Which comprises the step of producing an equal overload intelligibility spectrum.

2. The method of determining the most favourable energy level of speech at a transmitting point in a line Which comprises the step of determining for a small band of frequencies thereof under varying levels of the rest of the speech, the level which gives greatest intelligibility.

3. The method according to claim 2 Wherein the energy levels of a plurality of elementary bands of frequencies Within the speech range are each moved relatively to the level of the rest of the range, to determine for each band the level relationship giving greatest intelligibility, establishing this level relationship and thereafter if necessary adjusting the level of the entire range to produce under the established conditions the maximum of intelligibility.

4. The method of improving a transmission system subject to noise interference which comprises so designing the gain characteristic of the transmitting amplifier thereof with regard to the attenuation frequency characteristic of the cable and the spectrum of received noise interference that When the noise level is changed by equal infinitesimal amounts in any equal elemental frequency region the speech energy level for each fre- 

